The generation and recording of acoustic waves through a subterranean formation is a key measurement employed in wellbore logging. Acoustic waves propagate underground at velocities that vary from different geological formations. For example, an acoustic waves travel at about 4000 meters per second through sandstone, and about 5000 meters per second through limestone. Sound waves are generally classified into two types: longitudinal and transverse. A longitudinal or compressional wave is one in which the medium oscillates in the same direction the wave propagates. A transverse or shear wave is one in which the medium oscillates perpendicular to the direction the wave propagates. The speed or velocity of an acoustic wave through a medium is related to the porosity of the medium, an important characteristic of a formation insofar as its potential for containing hydrocarbons is concerned.
Many different logging tools are currently available for taking acoustic measurements to calculate acoustic wave velocity (and its reciprocal, slowness). A typical logging tool includes two receivers spaced along an axis of the tool at known distances from an acoustic transmitter. In the case of a single-transmitter, two-receiver tool suggested by the prior art, acoustic velocity is estimated by subtracting wave arrival times between the two receivers and dividing by the inter-receiver spacing. This estimate, however, is subject to inaccuracies due to tool tilt, borehole washouts, bed boundary effects, etc. Additional acoustic sources and receivers and more robust methods such as STC (Slowness-Time-Coherency analysis) among others have been used to reduce the inaccuracies introduced by such environmental effects. However, the more porous a formation is, the slower the wave propagation velocity is.
The foregoing logging techniques and others are often used in open and cased-hole wellbores, with an acoustic tool suspended on electric wireline. However, in recent years various tools and equipment have become available that enable logging-while-drilling. LWD systems are sometimes incorporated into special drill collars located near the bit. The results of logging measurements may be telemetered uphole, usually in the form of pressure pulses in the mud stream for detection, display and/or recording, substantially in real time or may be recorded for later retrieval. Usually the acoustic tool is equipped with a monopole transmitter for use in measuring compressional and shear wave velocity. However, shear wave velocity of a formation cannot be measured using a monopole transmitter when the shear wave velocity is slower than the velocity of the fluid (typically drilling mud) in the wellbore. This is because refraction cannot occur between the fluid and the formation if the velocity through the fluid is greater than the velocity through the formation (i.e. Vf>Vs).
In addition, LWD environments impose several harsh conditions and restrictions that an acoustic source transmitter must survive. The conditions and restrictions include, among other things, very limited space for a transmitter in the drill string, a need for high output pressure signals because of the noise generated by the drill, a highly erosive, high temperature and high pressure environment, and a limited power supply. There is a need for an LWD system capable of providing formation velocity under harsh conditions and restrictions, even when the formation velocity is slower than fluid velocity in the wellbore.